Collaborative vaccine clinical trials are now ongoing at the NIH and numerous Cancer Centers throughout the U.S. with vaccines developed in this program. The tumor-associated antigens targeted by these vaccines are (a) carcinoembryonic antigen (CEA), which is overexpressed on the majority of human carcinomas, (b) MUC-1, which is overexpressed on breast and other carcinomas, and (c) prostate-specific antigen (PSA). The genes for these tumor-associated antigens have been placed into two types of recombinant orthopox vectors: recombinant vaccinia and the replication defective avipox virus. Preclinical studies and clinical trials have now demonstrated the optimal use of these vectors in diversified prime and boost vaccine protocols. Immunoassays are currently being developed which detect and characterize T-cell responses to these tumor-associated antigens in vaccinated patients. It has now been shown that vaccinated patients can mount specific immune responses to known epitopes on these tumor antigens; the T cells generated can in turn kill tumor cells expressing these antigens. Agonist epitopes have now been identified for both CEA and PSA. Single amino acid changes in these epitopes have been shown to enhance the generation of cytolytic T-cell responses to both CEA and PSA expressing tumor cells. Studies are ongoing to develop more potent vector-based vaccines. Vectors have now been constructed which express three human costimulatory molecules as transgenes. Preliminary studies have shown that these vectors can be used to markedly enhance the activation of both na've and memory human T-cell responses. Studies are planned to translate these findings into new clinical vaccine trials.